As a headlamp of an automobile, there has been proposed an ADB light distribution control as one method for obtaining a light distribution for preventing dazzling to a vehicle (hereinafter referred to as a “front vehicle”) in a front region of an own vehicle, such as a preceding vehicle or an oncoming vehicle in the front region, while increasing an illumination effect of the front region of the own vehicle. The ADB light distribution control includes detecting a front vehicle by a vehicle position detection device, reducing or turning off a light amount in a region in which the detected front vehicle presents, while brightly illuminating other wide regions.
In recent years, the ADB light distribution control is also applied to a headlamp using a light emitting element such as an LED as a light source. Specifically, in the headlamp, light from a plurality of LEDs as light sources, that is, illumination region of respective LEDs are combined to form a light distribution for illuminating the front region of the own vehicle. Further, when a front vehicle is detected, LEDs in an illumination region corresponding to the detected front vehicle are dimmed or turned off.
In the ADB light distribution control, white light emitted from the plurality of LEDs is projected to the front region of the own vehicle by a projection lens to form a plurality of illumination regions, these illumination regions are combined and synthesized appropriately, and thus an appropriate illumination region is formed. However, a pattern shape of the light of the LEDs to be projected may vary due to aberration caused by the projection lens, which makes it difficult to perform the ADB light distribution control with high accuracy.
In JP-A-2017-16928, a rear main surface of the projection lens is designed to have a predetermined curvature, so that a direction of coma aberration is specified and uniformity of the light pattern to be projected is improved. However, since the technique of JP-A-2017-16928 does not cope with the aberration, this technique would not cope with change in pattern shape of the light caused by the aberration.
In order to cope with the aberration, it is considered to configure the projection lens with a plurality of lenses, for example, a triplet lens. In this case, in order to reduce the weight and cost of the projection lens, it is also considered to configure a part of the plurality of lenses with resin lenses. For example, JP-A-H8-68935 proposes, a technique in which in a camera including a triplet lens, a first lens and a second lens are formed of resin and a third lens is formed of glass.
Since the lens of JP-A-H8-68935 is applied to a camera which is often used at a so-called normal temperature (or room temperature), a problem caused by change in ambient temperature would rarely arise. However, a problem may arise when this type of lens is applied to a projection lens of a lamp of an automobile. That is, when applied to a projection lens of a lamp of an automobile, since an ambient temperature varies in a range of 0° C. to 80° C. while the lamp is turned on and turned off, change of optical characteristics of the triplet lens due to change of thermal expansion of the lens formed of resin, in particular a spot shape due to spherical aberration is noticeable. When the spot shape formed by the projection lens changes with the temperature change, the pattern shape of the illumination region to be projected also changes, and therefore, the reliability of the ADB light distribution control may deteriorate with the temperature change.